Hybrid electric vehicles and electric vehicles including, in a drive apparatus thereof, an electric motor which transforms electrical energy into mechanical energy have recently been of great interest as environment-friendly vehicles. The hybrid electric vehicles are now partially commercialized. Some types of hybrid electric vehicles employ a structure having a power supply apparatus driving and controlling a motor, the apparatus being provided with the ability of converting the level of a DC voltage input thereto for allowing a voltage applied for driving the motor (hereinafter referred to as “motor operating voltage”) to be adjustable according to operating conditions (e.g. number of revolutions, torque) of the motor, in order to highly efficiently drive the motor. In particular, the power supply apparatus is provided with a voltage step-up ability to make the motor operating voltage higher than the input DC voltage. Thus, a battery serving as a DC voltage source is reduced in size and the increased voltage allows the power loss to reduce, making it possible to improve efficiency of the motor.
For example, Japanese Patent Laying-Open No. 2003-244801 discloses a structure of driving and controlling an AC (alternating current) electric motor for driving wheels. In this structure, a DC voltage from a battery comprised of secondary cells is increased by means of a voltage step-up converter to generate a motor operating voltage, and the motor operating voltage is converted into an AC voltage by means of an inverter. With this structure, the voltage step-up ratio of the voltage step-up converter is set according to conditions of the motor and thus the motor can be operated highly efficiently.
For the aforementioned structure, however, a smoothing capacitor for stabilizing the motor operating voltage has to be provided on the output side of the converter which converts the level of the input voltage, as taught in Japanese Patent Laying-Open No. 2003-244801. Thus, when the motor operating voltage is changed according to operating conditions of the motor, the voltage held by the smoothing capacitor changes to cause stored electric power (P=C·V2/2) thereof to change.
Accordingly, when the motor operates under power running mode to convert electrical energy supplied thereto into mechanical energy (hereinafter referred to as “power running mode”) and an instruction to increase the motor operating voltage is issued according to increase in number of revolutions and torque of the motor, the stored electric power of the smoothing capacitor is accordingly increased. In the process of the increase in stored electric power of the smoothing capacitor, the converter supplies not only electric power to be consumed by the motor but also the electric power corresponding to the increase in stored electric power of the smoothing capacitor. Consequently, a situation where the converter outputs excessive electric power could be caused.
In particular, when the battery serving as the input voltage source has its power supply ability higher than the capacity of a switching device that constitutes the converter and thus the output electric power of the converter is limited by the capacity (electric current capacity) of the switching device, the switching device could be broken under the aforementioned situation, resulting in a failure in hardware.
On the contrary, when the motor operates under regenerative braking control to convert mechanical braking energy into electrical energy (hereinafter referred to as “regenerative mode”) and thereby supply the regenerative electric power from the motor to the battery and the motor operating voltage is decreased according to decreases in number of revolutions and torque of the motor, the regenerative electric power from the motor as well as electric power corresponding to the decrease in stored electric power of the smoothing capacitor are provided to the converter. Consequently, electric current passing through the switching device constituting the converter increases and the aforementioned situation could also be caused.